Chase Forecast for Holiday Weekend Tropical Storm

  Good evening everyone! We actually do have a little bit to cover tonight with this subtropical system in the Gulf of Mexico to isolated severe storms across the Plains, so let's do it. I'll start off by saying that I am packed up and ready to hit the road but I am still debating on whether I'm going South to cover the subtropical-low or heading West to chase occasional severe storms out there so today's chase forecast will simply break down all of it and take a look at all the options on the table. I'm probably targeting a marginal, but realistic tornado threat in the Gulf States as our subtropical system makes landfall. An areal tornado threat can't really be outlined at this point because there is too much uncertainty regarding where the storm goes but I expect at least some increase in tornado probs as we get closer to "go time" and he's making landfall. Enough of my rambling though and let's dive in to the interesting stuff...the analysis.

Subtropical Storm "Alberto"

Coincidentally, during the typing of this blog, the NHC issued advisory #1 at 15z and dubbed Invest 90L as "Alberto". This isn't unexpected as it has been organizing slowly for a few days and myself and others were eyeballing this potential for over a week now but nonetheless it means that things are developing as anticipated, at least up to this point. Alberto is currently located just East of the Yucatan peninsula of Mexico (19.5N 88.0W as of 13z). GOES East shows a broad area of convection in the Caribbean associated with this storm and the low-pressure circulation is more defined than it was even last night. It's not the tightest circulation by any means and the entire western half of the storm is fairly dry, only broken cumulus fields indicated the center if the low, but that is likely due to the land-interaction of the peninsula.

I expect conditions to become a little more favorable for organization over the next 36-48hrs, coinciding with Alberto's emergence into open water. When analyzing/forecasting tropical weather I prefer to assess water temperatures after I make an initial analysis & diagnosis of the storm itself using satellite imagery, radar, etc. (This is always preceded by the usual analysis and diagnosis of the atmosphere at-large) This is just the way I prefer to do it because, for one, it's fairly easy to analyze by simply looking at buoy obs and maybe some remote sensing data to fill in the gaps, but also the release of latent heat that occurs in tropical/subtropical storms over warm water is essentially the "fuel" that they use to survive & thrive.

Recent buoy observations, along with NOAA-19 and GOES satellite-derived data to fill in gaps where observations don't exist (errors occur due to cloud cover, etc) show SST's across the GofM in the 26C-28C range with a corridor of warmer water through the central Gulf and relatively cooler water eastward toward Florida. Some slight warming is possible with air temps potentially reaching 30ºC+, this could support a 1-2ºC increase. You generally want to see SST's of 27C+ to support tropical development, although like anything else in meteorology these values are just rough guidelines. The track of the storm will be key because it will determine if Alberto can hang out over the warmer open water or if it will be influenced by slightly cooler water near Florida. Our subtropical storm is currently parked in between two ridges with one to its West over Mexico and another off the East coast of Florida and these will be big players in steering Alberto as he drifts generally northward. The general motion should be to shoot the gap between these ridges, so to speak.

 Windsat satellite-derived wind fields. Errors can occur because this technology isn't perfect but areas of 30kts+ are evident.

Special "Nighttime Visible" satellite imagery from the NPP VIIRS satellite via the Naval Research Lab. Note the broad area of enhanced convection near the Yucatan peninsula.

Satellite-derived 6hr precipitation totals. Retrieved from the Naval Research Lab NexSat Program.

The above image shows the 700-850mb streamlines and is a good proxy for the steering layer of 1000mb-1010mb tropical cyclones. It seems reasonable that the Bermuda high will shift slightly eastward while Alberto drifts northward. I am not going to take a stab at landfall or anything yet and please forgive me for not explaining EVERYTHING that affects a tropical cyclone's track, like trochoidal motions, beta effects due to vorticity, and other factors like the Coriolis force, but the likely track seems to be through the central Gulf, although an embedded synoptic trough across the southern CONUS may draw the storm toward it, as often happens, so that is a variable worth monitoring. Either way, at this point I'm tentatively watching somewhere from the Florida panhandle to Louisiana. Heavy rainfall, strong winds, and dangerous rip tides will be likely along the West coast of Florida and the southern Alabama, Mississippi, and perhaps Louisiana coasts.

Now that I have a rough area for this thing to go let's look at how it may evolve. Wind shear has been relatively strong across the region, which has been part of the reason it was slow to organize, however it is expected to weaken somewhat. Honestly, the wind shear is probably going to be the main inhibiting factor going forward. We should see further intensification but I think the regional shear will be enough to mitigate a more intense storm. There will still be impacts and I don't think it's wise to write off any storm but it's just to say that I think this is one environmental influence that will prevent a more robust threat. 

I also assessed upper-level divergence and lower-level convergence. These factors tie in together and are related to mass-removal within the column of the cyclone. Continued removal of mass within the storm will result in pressure falls and I believe we will see a slow deepening/intensification. I'm not convinced we'll even see a sub 1000mb low yet but there appears to be enough ventilation to help sustain the cyclone as it moves northward to the CONUS.

Graph depicting output from 21 different numerical models suggesting possible trends in overall wind shear across the Gulf of Mexico in the coming days.

GFS output for wind shear over the next 30hrs. It does show a weakening of shear aloft but it's still painting a corridor of flow resulting in 30-40kts of deep-layer shear in some areas. I like to see roughly < 30kts to feel confident in increasing potential of a stronger tropical storm. This is not the "be all, end all" and as I said earlier, these guideline values are simply bases to refer to. Assigning any rigid, arbitrary "limits" to the fluid processes of which we're dealing with can and will get you in trouble at some point as nature doesn't recognize our thresholds. 

Another factor to consider is the possibility of tropical cyclone induced tornadoes. The SPC hasn't been able to delineate an areal risk for tornadoes associated with Alberto yet because of the variability in the track but I anticipate they will eventually. Tropical cyclones are often able to produce embedded tornadoes, particularly in the northeast quadrant. In the simplest terms, this results from favorable wind fields, especially in the low-levels where southeasterly flow and surface friction often enhances the probability for circulations to develop. You can usually identify enhanced hodograph curvature and favorable wind profiles in this region. This is something I've assessed when I've chased hurricanes/tropical storms in the past and it is currently the area I'm leaning most toward trying to identify and targeting for my chase this weekend. My initial target looks to be in the region of the extreme southeast Louisiana coast to the western tip of the Florida panhandle but that may change.

The GFS (and other guidance as well) picks up on this increased low-level shear and is painting an area of pretty high storm-relative helicity values (500+m^2/s^2 0-3km) in southern Mississippi. This isn't to show exactly where I think the greatest tornado potential will be but is just to demonstrate the enhanced low-level shear that often occurs in tropical cyclones.

So before we compare the model guidance with what I've put together here, let's summarize. I'm thinking our subtropical storm Alberto will finish crossing the Yucatan Peninsula of Mexico before emerging over open Gulf water at which point he'll drift northward, undergoing a slow deepening as he does. Some wobble is possible given mesoscale eccentricities and a nearby synoptic trough but overall it looks like a generally northward track. Heavy rainfall, flooding, gusty winds, and dangerous rip tides are likely this weekend along Florida's West coast and by late weekend/Memorial Day along the southern Gulf coast. Once making landfall it is uncertain how quickly Alberto, eventually an extratropical low, will phase with the mean westerly flow but heavy rainfall/thunderstorms will be possible across the southeast CONUS into the Tennessee Valley. 

"Spaghetti Plot" of numerous forecast models showing simulated tracks of our cyclone. It seems that some of the models are picking up on the tendency for tropical cyclones to be drawn toward synoptic troughs, which is producing decent clustering of tracks supporting my forecast, however, there is still considerable spread with just as many models diverging from that track too. There are several outliers which need to be noted as well. Remember, the trend is your friend! If the models are trending toward a certain outcome then there is always a reason why and it should be investigated further. That said, a solid conceptual model needs to exist and a forecast should ALWAYS come from detailed analysis, diagnosis, and prognosis with model data being used to help guide us along *after* we've already made our own assessments ofhow the atmosphere will evolve over space and time.   

Deterministic model guidance from the GFS and even the NAVGEM. These are only isolated model graphics but they are a good proxy for the forecast I've come up with because they have "Alberto" taking a slight northwest jog before landfall and that's the way I'm leaning. I don't think this is going to be such a strong storm that it shows a lot of "deviant motion" so it seems it will be even more susceptible to mid-tropospheric flow and features. Like I mentioned earlier, given the tendency for these cyclones to be "drawn in" toward synoptic troughs I am anticipating that the trough to its northwest will influence its track. There's also a tendency for tropical cyclones to propagate toward areas where they are producing maximum latent heat release and that could play a role in this outcome too.

Above is the GFS output for total QPF through Tuesday evening (7pm CT). This is just to show the potential for flooding in many locations because widespread heavy rain is likely regardless of how "Alberto" evolves, although areas that see a more direct hit will of course see higher precip totals. After landfall the remnant extratropical-low will likely still be responsible for rainfall across the TN Valley but I didn't go past 102hrs because, 1: a passing frontal system begins to influence QPF totals in the output and I haven't assessed this system much yet so I don't want to lend any credence to it until I have, and 2: after 96hrs I start questioning the legitimacy of the GFS even more than usual because it starts to show the effects of mounting uncertainty stemming from each previous forecast hour (related to "chaos theory")

That will do it for tonight, I'll post updates throughout the weekend because at this point we're in the monitoring stage. It's going to be all about tracking in real-time as "Alberto" emerges over the open Gulf, plus we'll have more Air Force recon data to work with so that'll help. I said several days ago that this system could flirt with category 1 hurricane strength, and even though confidence is higher that it maintains tropical storm intensity, I still think there's an outside chance that it can if it manages to stay over the central Gulf where the longer residence time over warmer water may compensate for the influence of shear. Regardless of what anybody calls it, do not ignore this storm if you're in any if the threatened zones. There will be flooding risks, strong wind gusts, and dangerous ocean currents. Simply monitor official sources for information, assess your situation, and be smart. I'll get into the upcoming severe risk in the northern plains and upper Mississippi River Valley with tomorrow's post.
Stay cool folks, the next adventure is right around the corner.
DKK